Process for producing boron nitride felt

ABSTRACT

A novel process for producing a thin flexible self supporting boron nitride felt comprising the following steps .[...]..Iadd.: .Iaddend. 
     A. purifying boron nitride fiber by washing with water and subsequently extracting with an aliphatic alcohol containing from 1 to 3 carbon atoms.Iadd.;.Iaddend. 
     B. forming a mat from the fibers in step A and incorporating therein an aqueous solution of an inorganic water soluble binder selected from the group consisting of water soluble halides, nitrates, nitrites and carbonates of the alkali metals, the alkaline earth metals, the Group III A metals and their mixtures; and 
     C. drying the resulting binder.Iadd.-.Iaddend.containing mat obtained from step (B) at a temperature below the melting point of the salt.

This invention relates to a method for preparing an ion-permeable,electron-insulating separator for use in an electrical energy storagedevice. More particularly this invention relates to a method forpreparing a separator composed of boron nitride felt which isparticularly useful in electrical energy storage devices such aselectrochemical cells under the most chemically severe operatingconditions. A separator of this type constitutes an effective barrierbetween electrodes of opposite polarity and provides a means for spacingthe electrodes.

Under operating conditions in some electrochemical cells andparticularly in cells employing fused salt electrolytes, the electrodesundergo distortion as is evidenced by their buckling and swelling.Heretofore the distortion was compensated for by spacing the electrodesfar enough apart so that even after buckling and swelling the electrodesfailed to contact one another. Thus by one method of the prior artshort-circuiting between electrodes was avoided. However, such excessivespacing was both wasteful and inefficient.

The general requirements of the separator of the present invention arethat it be as thin as possible to minimize electrical resistance and toallow for compact electrical energy storage device, and that it possessthe mechanical strength to withstand operating conditions entailingcharging and discharging of the cell. Because electrochemical reactiondevices such as electrochemical cells herein described operate over awide temperature range from about ambient to 750°C and above, dependingon the electrolyte, the internal components are subject to considerablestress in the form of expansion and contraction. Therefore, theseparator of the present invention must possess the structural integrityto withstand wide ranges of operating conditions. The separator mustalso possess insulating properties to prevent the short-circuiting ofthe electrodes should a bridge form from materials capable of promotingthe formation of short-circuits. Also the separator must have a maximumporosity and must accommodate ionic diffusion. Any hindrance of the ionsin their movement affects the efficiency of the electrochemical device.Of course, the porosity of the separator desirably approaches 100percent.

The boron nitride separator prepared by the process of the presentinvention is designed for use in a highly corrosive, high-temperatureenvironment, and it therefor is manifested that it have a high meltingpoint, and be chemically and thermally inert, stable, and insolubleunder operating conditions.

Boron nitride bulk fiber as obtained from commercial sources is eitherin the form of a loose, fluffy fiber or is in the form of roving, bothof which are difficult to handle and to convert into a reproducible feltform. Although boron nitride felt is available commercially, the felt isunsatisfactory from the standpoint of impurities introduced in felting.By the process of this invention a minimum of impurities is introducedin the processing steps, and the roving or loose, fluffy boron nitridefiber can be readily converted into a flexible, integral,self-supporting felt of high purity that is suitable for use under theoperating conditions heretofore specified. This is accomplished byemploying an inorganic salt or combination of salts as a binder for theboron nitride fibers in the mat. Preferredly for practical purposes, theinorganic salt or salts utilized in this process are the more common,water soluble salts wherein the cation may be an alkali metal, analkaline earth metal or a Group III A metal and the anion is a halide,nitrate, nitrite, or certain carbonates. In addition to the use of asingle salt as a binder material, binary and ternary salt mixtures canalso be utilized, such as lithium chloride-potassium chloride, potassiumiodide-lithium iodide, potassium chloride-magnesium chloride, magnesiumchloride-sodium chloride, lithium bromide-potassium bromide, calciumchloride-lithium chloride, lithium fluoride-rubidium fluoride, magnesiumchloride-rubidium chloride, aluminum chloride-lithium chloride, andmixtures thereof. Examples of ternary mixtures are lithiumchloride-potassium chloride-cesium chloride and lithiumbromide-potassium bromide-lithium chloride. A preferred bindercomposition is a eutectic mixture of potassium chloride and lithiumchloride.

The separator of this invention is particularly suitable for use infused salt batteries wherein the binder may have the same composition asthat of the electrolyte of the electrochemical cell. It is also feasibleto employ the binder-saturated boron nitride felt as the onlyion-containing and conducting medium in the battery, wherein the felt isplaced between and is wrapped around the electrodes.

In the broadest aspect of this invention, the process for preparing thefelt comprises adding an aqueous suspension of boron nitride fiber to asaturated solution of the binder in water, stirring the resultingmixture to obtain a uniform suspension of the fiber in water, separatingthe fiber-salt mixture from the aqueous medium as by filtration or bysome other known means to produce a felt-like sheet, and subsequentlyshaping and drying the sheet. Alternatively, in a more preferredprocedure, an aqueous suspension of the boron nitride fiber is filteredto produce a thin fiber mat, an aqueous solution of the binder issprayed onto the surface of the mat, and the mat is then shaped anddried to obtain an integral, flexible, spacer. This latter procedureenables better control of the binder composition and concentration inthe felt, particularly where the binder comprises a mixture of salts incertain definite proportions. On slow drying, the fiber mat may bereshaped periodically to adjust the form and thickness to the desireddimensions.

For satisfactory performance in an electrical energy storage battery,the boron nitride fiber employed in preparing the felt of this inventionshould contain a minimum of foreign matter so as not to contributeexcessively to the leakage current of the electrochemical cell. It istherefor preferred that the boron nitride fiber contain less than aboutone percent by weight of any foreign substance. Boric oxide is a commonimpurity found in the fibers and this can be readily removed byconverting the boric oxide to boric acid by means of washing with waterand removing the boric acid by extraction with a low molecular weight(C₁ to C₃) aliphatic alcohol. It is also advantageous, but not essentialto use fibers with a diameter of ≦ 10 microns, and a density of ≧ 1.85grams/cc of fibers, and a d₀₀₂ spacing ≦ 3.41A.

Structural variables of the felt such as orientation of fibers,uniformity of fiber density, and thickness, are largely controlled bythe concentration of fiber in the aqueous suspension. By using moredilute suspensions of the fiber, it is possible to obtain felt withuniform density, minimum orientation and a greater degree ofcross-linking required for strength and flexibility to survivesubsequent handling. With the use of a more dilute suspension it is alsopossible to produce felt with a minimum of thickness which is importantfrom the standpoint of space conservation in electrochemical cells. Forexample, we have found that felts with satisfactory physical propertiesand having dimensional thicknesses of from 15 to 75 mils can be obtainedby employing concentrations of boron nitride fiber of from about 1 to 5grams per liter of water. It is also preferred, but not essentialhowever, for a mat in this thickness range to have a fiber density offrom about 2.5 to about 12.5 × 10⁻ ² g/cm² of geometric surface area offelt. It has been found that lower drying rates also contribute toreduction of separator thickness, and that very satisfactory results areobtained by slow drying under reduced pressures at temperatures rangingfrom about 50°C to a temperature just below the melting point of thebinder.

The concentration of the binder in solution necessary to producesatisfactory felts may vary with the salt employed and the manner inwhich it is incorporated into the boron nitride fiber. On adding anaqueous suspension of the fiber to a solution of the binder, it ispreferred to employ a saturated salt solution, and the concentrationwill vary with temperature and the solubility characteristics of theparticular salt employed. If, however, the salt is applied by sprayingon to a preformed mat of fibers, generally more dilute salt solutionsare required. It is preferred that the final concentration of the binderin the mat range from about 2 × 10⁻ ² to about 5.5 × 10⁻ ² g/cm² of mat.

It has also been found to be advantageous to produce multilayerlaminates of boron nitride felt by combining several layers to obtainfelt with increased cross-linking of fibers and to compensate for minorvariations in thickness.

The following examples represent preferred modes of carrying out theprocess of this invention, but the scope of the invention is not limitedto these procedures.

EXAMPLE 1

Twenty-five grams of boron nitride roving (Carborundum, high puritytextile roving) were added to 500 mls. of distilled water. The slurrywas boiled for 15 minutes and the water decanted. This latter step wasrepeated two additional times in order to convert boric oxide present asan impurity in the fiber to boric acid. The boric acid was then removedfrom the boron nitride roving by soaking in 500 mls. of methanol for 15minutes and decanting the methanol. The boron nitride fiber was thenplaced in a Waring three-speed commercial blender containing 3,500 mls.of distilled water, and the fibers were blended for one second. Theblended mixture was then diluted with distilled water to a total volumeof 12 liters and agitated by means of an air sparger. The resultingsuspension was filtered by vacuum through a porous filter with a nickelsurface and having 3.8 μ holes and 40 percent void fraction, and thefiber mat was checked for uniformity and density. The mat was removedfrom the filter with the aid of a gentle stream of air directed to theback of the filter and cut to the appropriate shape and size requiredfor the separator. Excess water was removed from the mat by pressing,and the mat was sprayed with a binder solution consisting of a mixtureof 7.3 grams of potassium chloride and 5.8 grams of lithium chloride in52 mls. of water. The final mat containing 2.54 × 10⁻ ² g/cm² of binder,was then dried in a hot air oven at 50°C for one hour and reshaped to amold block. The mat was returned to the oven and dried at 125°C, andthen placed in a vacuum chamber and dried for a period of about 15 hoursduring which time the temperature increased to 300°C and the pressurewas reduced to approximately 10⁻ ⁵ Torr. The final boron nitride mat hada uniform thickness of 0.030 inches.

The resulting felt was utilized as a separator between the electrodes inan electrochemical reaction cell comprising a high surface area carbonelectrode and an opposing electrode composed of an aluminum-lithiumalloy. The space between the electrodes provided a clearance of 0.030inches. The electrodes were immersed in an electrolyte composed of aeutectic mixture of lithium chloride and potassium chloride, (59 molepercent LiCl and 41 mole percent KCL), and the cell was operated at atemperature of 500°C. The cell was contained in a rectangular 1,008carbon steel housing, 6 inches wide, 8 inches high, and 1 inch thick,with the fused salt about 1/2 inch from the top of the cell. The cellwas sealed to prevent liquid and gas evolution and was provided with apositive current carrier. The cell performed in the rechargeable mode asa secondary sealed system. The cell was cycled for 30 days between 3.34v and 1.0 v. The separator was inspected after completion of the testand was found to be in excellent condition.

Example 2

The procedure of Example 1 was repeated using a eutectic mixture oflithium bromide and potassium bromide (mol ratio 1.63 LiBr/1KBr) as theelectrolyte and as the binder in the boron nitride felt. The fiberdensity of boron nitride in the final felt was 8.10 × 10⁻ ² g/cm² ofmat, the felt had a thickness of 0.050 inches, and it had a bindercontent of 4.0 × 10⁻ ² g/cm² of mat. The cell was cycled as in Example 1for a period of 30 days and on examination of the separator oncompletion of the test the separator was found to be in good condition.

Example 3

The procedure of Example 1 was repeated using lithium chloride-potassiumchloride eutectic as the electrolyte and cesium chloride as the binderin the boron nitride felt. The felt had a fiber density of 4.05 × 10⁻ ²g/cm² of mat, a thickness of 0.025 inches, and it had a binderconcentration equivalent to 2.0 × 10⁻ ² g/cm² of mat.

When employed as a separator in an electrical energy storage cell as inExample 1, which was operated for 20 days, the separator was found to bein excellent condition.

Example 4

The procedure of Example 1 was repeated except that the binder in theboron nitride mat consisted of a mixture of lithium chloride, rubidiumchloride and potassium chloride in a molar ratio of 2.55:1.18:1. The mathad a thickness of 0.015 inches, a fiber density of 2.43 × 10⁻ ² g/cm²of mat, and a salt concentration of 5.1 × 10⁻ ² g/cm² of mat. Theseparator was wrapped around both electrodes of the cell, and the onlyelectrolyte present in the system was held interstitially by theseparator. After cycling the cell for 15 days, the separator showed nosigns of deterioration.

We claim:
 1. A process for producing a thin, flexible, integral boronnitride felt comprising the following steps:A. purifying boron nitridefiber by washing with water and subsequently extracting with analiphatic alcohol containing from one to three carbon atoms; B. forminga mat from the fibers in step (A) and incorporating therein an aqueoussolution of a binder .[.in an amount such that the final concentrationof the said binder in the mat ranges from about 2 × 10⁻ ² to about 5.5 ×10⁻ ² g/cm² .]., said binder being an inorganic salt selected from thegroup consisting of water soluble halides, nitrates, nitrites andcarbonates of the alkali metals, the alkaline earth metals, the GroupIII A metals, and their mixtures; and C. drying the resultingbinder-containing mat obtained from step (B) at a temperature below themelting point of the salt.
 2. The process in claim 1 wherein the binderin step (B) is incorporated by adding an aqueous suspension of the boronnitride fiber to a saturated aqueous solution of the binder, and formingthe mat therefrom.
 3. The process in claim 1 wherein the binder in step(B) is incorporated by spraying the mat formed from boron nitride fiberwith an aqueous solution of the binder.
 4. The process in claim 1wherein the inorganic salt consists of a eutectic mixture of lithiumchloride and potassium chloride.